1. Field of the Invention
The present invention relates to a defect inspecting apparatus and a defect inspecting method, and in particular, to defect inspecting apparatus and method that inspect defects on a mask pattern, the mask pattern being formed on a mask such as a reticle.
2. Description of the Related Art
With an increase in the degree of integration of semiconductor devices such as LSIs, efforts are being made to reduce the sizes of mask patterns formed on masks such as reticles. This requires a high performance to be exhibited by defect inspecting apparatuses that inspect mask patterns for defects. Jpn. Pat. Appln. KOKAI Publication No. 10-177246 proposes a defect inspecting apparatus that detects defects by using an image pickup sensor such as a CCD to pickup an image of a mask pattern and comparing the picked-up image with a reference image.
Possible methods for improving the resolution of the defect inspecting apparatus include those for increasing the magnification of a detection optical system and those for increasing the number of pixels in the image pickup sensor. An increase in the magnification of the detection optical system reduces the area of a field region on a surface of the mask on which the pattern is formed. This increases the number of images picked up by the device, necessarily reducing the throughput of defect inspections. On the other hand, an increase in the number of pixels in the image pickup sensor enlarges an illumination field illuminated by an illumination optical system. This hinders the efficient use of illumination light rays. This problem is particularly serious if an image pickup sensor such as a TDI (Time Delay Integration) operation type is used which has a vertically long shape. For example, it is assumed that an increase in the number of pixels has doubled the vertical size of the image pickup sensor. Then, since the illumination optical system normally illuminates a circular illumination field, the area of the illumination field must be quadrupled. This necessarily increases the intensity of an optical beam from the illumination optical system that illuminates the regions of the image pickup sensor except the field region. As a result, the illumination optical beam may become wasteful; the effective use of the illumination optical beam is hindered.
It is important to increase the resolution of the defect inspecting apparatus in order to improve its performance. However, as described above, the improvement of the resolution involves problems such as a decrease in throughput and an impediment to the effective use of the illumination optical beam. Therefore, it is not easy to improve the resolution of the defect inspecting apparatus.